1. Field of the Invention
The present invention relates to an optical transmission apparatus. More particularly, the invention relates to a wavelength division multiplexing optical transmission apparatus that uses wavelength division multiplexing (WDM) for transmitting large amounts of data at high speed through a single optical fiber by multiplexing/demultiplexing optical signals of different wavelengths, and specifically to a wavelength division multiplexing apparatus in which the stability and accuracy of a filter bandwidth for each signal is increased for higher density wavelength multiplexing.
2. Description of the Related Art
FIGS. 1A to 3 are diagrams showing one configuration example of a multiplexer/demultiplexer in a prior art wavelength division multiplexing optical transmission apparatus.
The example shown in FIG. 1A uses an arrayed-waveguide grating (AWG) 10, the dominant type of optical multiplexer/demultiplexer in use today. The AWG 10 functions, like a diffracting grating, using interference of diffracted light beams from a plurality of waveguide arrays of different lengths, and is applied to various devices such as a wavelength combiner/splitter, a wavelength router, etc.
In wavelength division multiplexing, the AWG 10 takes different frequency components as inputs from a plurality of input ports and combines them for output through a single output port. Generally, the AWG 10 has an n×n frequency switching function, with n input ports and a matching number, n, of output ports, as shown in FIG. 2, and the frequency component from each input port is output through each corresponding output port. Since the above wavelength division multiplexing does not require all the output ports, only one of the output ports is used (signals λ1 to λ4 shown within the dashed lines on the input port side in the figure are output as a wavelength division multiplexed signal, λ1 to λ4, shown within the dashed lines on the output port side).
Here, as shown in FIG. 3, the AWG 10 is generally fabricated as a wavelength combiner/splitter comprising two slab waveguides 18 and 19, having collimating and converging lens functions, integrated on a single substrate 17. The optical filter characteristics between the input and output ports of the AWG 10 have temperature dependence, the parameter being the length of each waveguide, so that the filter bandwidth fluctuates as the waveguide expands or shrinks due to changes in temperature. The fluctuation is the same for each channel, and a wavelength shift manifests itself as the same vector change on all channels.
Therefore, the AWG 10 incorporates a temperature control circuit 11 in order to stabilize the filter characteristics at the specified wavelength. FIG. 1B shows a prior art configuration example of the temperature control circuit incorporated in the AWG. In the example shown here, a sensor resistor 15 having a stable resistance temperature coefficient and a heater resistive element 16 for generating heat proportionally to power consumption are mounted within the AWG, and further, circuits 13 and 14 for temperature control are provided that detect a change in the resistance of the sensor resistor 15 and supply current to the heater resistive element 16.
However, since its component parts themselves are subject to initial variations and other characteristic degrading factors such as temperature variations and aging, the prior art temperature control circuit, 13 to 16, has had the problem that if the initial variations existing in the component parts can be accommodated at the time of initial setting, there is no way to cope with the fluctuation of the filter center wavelength that may occur due to temperature variations of the parts, aging of the AWG, etc. during operation thereafter. As a result, the wavelength division multiplexing optical transmission apparatus has had to be designed by also considering wavelength stability degrading factors such as temperature characteristics and aging, and this has been one of the great barriers to the development of higher density wavelength division multiplexing optical transmission apparatus.